The invention relates to an oven for sealing a panel to a funnel, thus forming an envelope suitable for use in a display tube, the oven comprising a tunnel structure, means for heating and circulating gas(es) inside the tunnel structure, and at least one mount for conveying an assembly of a panel and a funnel through the tunnel structure.
The invention also relates to a process for sealing a panel to a funnel, which involves heating and subsequently cooling an assembly of a panel and a funnel at least by means of gas(es) circulating in an oven and flushing the interior of the assembly with a fluid, as well as to the use of an in-line exhaust oven for sealing a panel to a funnel.
A display tube, such as a color cathode-ray tube, usually comprises a bulb or envelope which is composed of a panel or display screen and a funnel or cone which are adhered together. The funnel is accurately positioned in an adhering jig and the edge of the funnel is provided with a glass frit. The panel is placed on top of the funnel and the adhering jig containing the assembly of a panel and a funnel is passed through an oven in which the assembly is heat-treated and the glass frit recrystallizes. Thus, the panel and the funnel are joined rigidly and in a vacuum-tight manner.
Subsequently, an electron gun is placed in the neck of the funnel, and the envelope is evacuated by passing it once again through an oven in which the display tube is degassed at a fixed temperature. During the cooling process, the envelope, which is now usually referred to as display tube, is sealed in a vacuum-tight manner.
An example of a conventional process for forming envelopes suitable for use in display tubes is disclosed in e.g. U.S. Pat. No. 5,277,640. This publication describes a frit seal furnace (1), comprising a furnace body (3) having heating means (2) such as tube burners, and conveyor belts (5) movable in the furnace body (3) (The numbers between parentheses in this and the next paragraph relate to the numbers in the Figures of U.S. Pat. No. 5,277,640.). The furnace body (3) is lined with a heat insulation (6), and the heating means are disposed in aligned positions on opposite sides of the conveyor belts. Fans (7) are disposed above the conveyor belts (5) for directing air downwardly in the furnace. The heating means are divided into zones (8) arranged along the direction in which the belts move. Each conveyor belts comprises a pair of chain belts (4) and can be driven in a circulating fashion, over and below a furnace floor (3a). In comparison with a mesh belt, said chain generates fewer metal particles.
A panel (31) and a funnel (32) are superposed one on the other with a frit glass (33) interposed therebetween in a mount (21). This mount comprises a base (22) in the form of a frame, a holder (24) connected to the base by a plurality of legs (23) for engaging a round portion of the funnel, and abutments (25) for abutting against three sides, respectively, of the panel and the funnel.
The mount also has a mesh screen as a floor surface, i.e. a surface where the mount is placed on one of the conveyor belts in the frit seal furnace.
U.S. Pat. No. 5,277,640 further mentions that, since air is caused to flow downwardly in the furnace by the fans, any metal particles are prevented from being attached to (the outer surface of) the frit glass, and that it is possible to insert an air nozzle into the assembly from below in the frit seal furnace. Thus, clean air may be supplied from an external source through the air nozzle into the assembly. However, insertion of the nozzle into the assembly is very difficult or even impossible in practice due the presence of the aforementioned conveyor chains, the legs, and the mesh screen.
A disadvantage of this oven and process is that a substantial number of sealed envelopes still do not fulfil the specifications for a commercial television set. In particular, one or more pixels of the panel may be rendered inoperative by the presence of metal particles.
It is an object of the present invention to further reduce the number of metal particles generated by the means with which the assemblies of a panel and a funnel are transported through the tunnel structure. It is a further object of the present invention to facilitate the supply of a fluid to the interior of the assembly and hence allow more controlled and effective flushing.
To this end, the oven as described in the opening paragraph is characterized in that the tunnel structure is provided with a longitudinal slot and in that the mount comprises first components for carrying an assembly of a panel and a funnel, which first components, at least during sealing of the panel to the funnel, are placed inside the tunnel structure, and second components for guiding the first components through the tunnel structure, which second components are placed outside the tunnel structure and support the first components, via the longitudinal slot.
It appeared that many defects in the panel or frit seal originate from pollution of the interior of the envelope by dust particles, such as metal particles and small fragments of glass, from the oven. By means of the invention, the greater part of the wear and hence of the number of metal particles generated is located outside the tunnel structure and contamination is effectively reduced. Further, fluid communication between the interior of the assembly and a fluid source, such as filtered clean air, outside the oven can be established through the longitudinal slot.
Accordingly, it is preferred that said second components comprise a means for flushing the interior of the assembly with a fluid that is obtained from a source that is substantially separated from the gas(es) circulating in the oven. It is further preferred that said means for flushing the interior of the assembly comprises at least one pump or compressor, which is in fluid communication with the interior of the assembly. In a very practical embodiment, the factory hall itself serves as a source of the flushing fluid, especially when the air in the factory is filtered and/or conditioned. It that case care should of course be taken that the inlet of the means for flushing the interior of the assembly is not too close to the factory floor or, e.g., wheels or a guiding rail, unless a filter of some sort is provided downstream of the inlet.
The present invention further relates to a process as described in the opening paragraph, which preferably employs an oven as described above, wherein the fluid, which primarily serves for flushing the interior and removing gases, in particular NOx, generated by the frit and by the electrically conductive layer of the funnel, is also employed to control the temperature of the assembly during heating and/or cooling. Thus, the temperature treatment of the assembly can be carried out more homogeneously and/or heating and/or cooling can e.g. be accelerated as a result of which relaxation of built-in pre-tensions, which are, inter alia, used to reduce the risk of a cracked envelope, is suppressed. The presence of oxygen in the fluid will prevent or at least suppress the chemical reduction of metal oxides, such as PbO, which is usually a major component of the glass frit, or Fe3O4, which is frequently used in a composition for forming an electrically conductive layer inside the envelope. The use of a fluid as described above is not limited to use in an oven according to the invention. The process may be used in any situation wherein the interior of an envelope is flushed.
The invention can be implemented by using an in-line exhaust oven for sealing a panel to a funnel, which oven comprises at least one mount for conveying an assembly of a panel and a funnel through the tunnel structure, wherein the mount comprises first components for carrying an assembly of a panel and a funnel, which first components, at least during the sealing of the panel to the funnel, are placed inside the tunnel structure, and second components for guiding the mount along and through the tunnel structure, which second components are placed outside the tunnel structure and comprise a means for flushing the interior of the assembly with a fluid. An oven suitable for such use is, for instance, described in U.S. Pat. No. 4,498,884 (see FIG. 4 and the accompanying description in that application).
It is preferred that the second components comprise a means for flushing the interior of the assembly with a fluid obtained from a source which is substantially separated from the gas(es) circulating in the oven.